Process and compositions for the recovery of ascorbic acid

ABSTRACT

The invention provides an extraction process for the recovery of ascorbic id from an aqueous feed solution containing the acid at a concentration of less than 0.7 mol/kg.

This application is a 371 of PCT/US96/08093 filed May 31, 1996.

The present invention relates to a process for the production ofascorbic acid. More particularly, the present invention relates to therecovery of ascorbic acid from aqueous solutions containing the same indilute concentrations.

As described, e.g., in Kirk-Othmer's Encyclopedia of ChemicalTechnology, Third Edition, ascorbic acid (L-ascorbic acid,L-xylo-ascorbic acid, L-threo-hex-2-enonic acid τ-lactone) is the namerecognized by the IUPAC-IUB Commission on Biochemical Nomenclature forvitamin C. The name implies the vitamin's antiscorbutic properties,namely, the prevention and treatment of scurvy. L-ascorbic acid iswidely distributed in plants and animals. The pure vitamin (C₆ H₈ O₆,mol. wt. 176.13) is a white crystalline substance derived from L-gulonicacid, a sugar acid, and synthesized both biologically and chemicallyfrom D-glucose. ##STR1##

Although natural and synthetic vitamin C are chemically and biologicallyidentical, in recent years a limited amount of commercial isolation fromvegetable sources, e.g., rose hips, persimmon, citrus fruit, etc., hasbeen carried out to meet the preference of some persons for vitamin Cfrom natural sources. L-ascorbic acid was the first vitamin to beproduced in commercial quantities, and manufacture is based on thewell-known Reichstein and Grussner synthesis, which involves the stepsof hydrogenation of D-glucose to D-sorbitol; fermentation (oxidation) toL-sorbose; acetonation to bis-isopropylidene-α-L-sorbofuranose;oxidation to bis-isopropylidene-2-oxo-L-gulonic acid, and hydrolysis,rearrangement and purification to L-ascorbic acid.

A direct fermentation of glucose to ascorbic acid would be veryattractive, saving on operations and on expensive reagents, in additionto its being derived from a natural fermentation process, as opposed toa synthesis involving chemical steps. There are indications that suchdirect fermentation to ascorbic acid is feasible. Yet industrialproduction of ascorbic acid through direct fermentation seemsimpractical, in view of the low product concentration in thefermentation liquor, which normally is in the range of less than 0.7mol/kg. Purifying the ascorbic acid by conventional methods would resultin a purified product of concentrations similar to those in thefermentation liquor. Due to its high solubility in water, the cost ofascorbic acid crystallization by water evaporation would be prohibitive.

Several methods were proposed for combining purification of carboxylicacids with their concentration. In the case of citric acid, it isachieved by the addition of lime to crystallize calcium citrate, whichhas very low solubility in water. This salt is separated, washed andacidulated with sulfuric acid. Purified and concentrated citric acid isobtained. This method is not applicable for ascorbic acid, as its alkaliand alkali earth salts are highly soluble.

A process was proposed in which carboxylic acids were extracted and thendisplaced from the extractant by a solution of concentrated mineralacids. Both liquid (long chain amines) and solid (resins carrying aminegroups) anion exchangers could be considered for this purpose. Thepurity of the displaced carboxylic acid depends on the preference of theextractant to the mineral acid. Such a process might be applicable forascorbic acid separation and concentration, provided that the extractantis strong enough to reach high extraction yield, that it shows highpreference to the displacing acid, and that the ascorbic acid is stableat the high acidity of the displacing solution.

The regeneration of the anion exchanger would require neutralization bya base. Using HCl as the displacing acid and distilling it of theextractant was proposed, but the high temperatures required and theextractant's decomposition at these conditions are prohibitive. If theanion exchanger is represented by B, the ascorbic acid in thefermentation liquor and in the pure form are AA_(F) and AA_(P),respectively, the displacing acid is HCl, and the neutralizing base isNaOH, the equations of the process stages and of the overall reactionare as follows:

    B+AA.sub.F →B.AA

    B.AA+HCl→B.HCl+AA.sub.P

    B.HCl+NaOH→B+NaCl+H.sub.2 O

    AA.sub.F +HCl+NaOH→AA.sub.P +NaCl+H.sub.2 O

Reagents are consumed, and a by-product salt of no (or negative) valueis produced.

Thus, despite the widely felt need for a more attractive process to meetthe exceedingly high demand for ascorbic acid, to date no such processhas been proposed or commercialized.

In 1976, there issued British Patent 1,426,018 and in 1981 there issuedthe corresponding U.S. Pat. No. 4,275,234, directed to the recovery ofacids from aqueous solutions. In said patents, there are exemplified therecovery of citric acid, lactic acid, oxalic acid, and phosphoric acidfrom an aqueous solution of the same acid; in fact, said U.S. Patent isspecifically limited in its claims to the recovery of one of said fouracids.

While the process of the present invention as defined herein formallyfalls within the scope of said aforementioned British patent, therelevant teachings of which are incorporated herein by reference, and inthis sense constitutes a selection therefrom, as will be explainedfurther below, not only do said patents neither teach, suggest, norexemplify the applicability of said process to the recovery of ascorbicacid, but in fact, from a careful analysis of said patents, one wouldnot expect said process to be feasible for the recovery of ascorbicacid, as is also evidenced by the fact that nineteen years have passedfrom the publication of said British patent without any person skilledin the art either suggesting or applying said process to ascorbic acidrecovery.

Referring now to said patents and the teachings thereof, one finds thatthe process taught therein utilizes the effect of temperature onphosphoric and carboxylic acid extraction by amine-based extractants.The term "amine" as used herein means water-immiscible amine, with atotal of at least 20 carbon atoms on its chains. Said patents teach thatsuch amine-based extractants (ABE) lose much of their extractionefficiency upon temperature elevation. This loss of efficiency isreferred to as "temperature sensitivity of extraction" (TS). Themagnitude of this TS can be represented by the ratio of the distributioncoefficient at the lower temperature (D_(T1)) and the distributioncoefficient at the higher temperature (D_(T2) ). High TS provides forthe purification and the concentration of carboxylic acids throughaltering the temperature between extraction and back-extraction. Theacid is extracted from the fermentation liquor by an ABE at lowtemperature, and is then back-extracted with water at an elevatedtemperature. The aqueous solution obtained from that back-extraction is,in many cases, more concentrated than in the fermentation liquor. Thisprocess is referred to herein as the "temperature swing process" (TSP).The attraction of such processes is in the fact that the sole energyconsumption is that of sensible heat, which saves a lot of the latentheat of water evaporation in the final concentration.

As explained in U.S. Pat. No. 4,275,234:

"The concepts of "lower temperature" and "higher temperature" are notunderstood in absolute terms. What matters . . . is the temperaturedifferential. This will have to be at least 20 degrees (centigrade),both for operation convenience and in order to make both the extractionand the back-extraction as complete as possible. The extraction may becarried out at temperatures as low as near the freezing point of theaqueous acid solution and the temperature of the back-extraction may beat or near the boiling point of the extract or the water at atmosphericpressure, or if the back-extraction is carried out under elevatedpressure, at an even higher temperature, always on condition that thetemperature and pressure are so chosen that the amine remains in theorganic phase. In many cases the extraction can be carried out at ornear room temperature, and the stripping operation at a temperature ofabout 20 to 40 degrees (Centigrade) above room temperature. As a rule,the stripping operation is the more effective, the higher the strippingtemperature, but the extraction and stripping temperatures will beselected in individual cases in accordance with practical factors, suchas corrosion-resistance and the costs of the equipment, costs of heatingand cooling of the streams of the acid solution, the extract and theextractant, the required concentration of stripped acid, etc.

"If the aqueous liquid used for stripping the extract is water, theback-extract is an aqueous solution of the free acid. If desired, theback-extracting operation may be so conducted that the back-extract isan aqueous solution of a salt of the extracted acid. For example,back-extraction with an aqueous alkali metal (in this context "alkalimetal" includes ammonium) hydroxide solution yields an aqueous solutionof the corresponding alkali metal salt of the extracted acid. Or theaqueous back-extracting liquid may be, for example, an alkali metalchloride solution. In this case, too, the back-extract contains thecorresponding alkali metal salt of the extracted acid while the amine inthe extractant is converted into its hydrochloride. This will thus haveto be decomposed, e.g. by treatment with calcium hydroxide, forreconstituting the extractant. Sometimes it is advantageous to performfirst a back-extraction with water in order to recover the major part ofthe acid in the free state. The residue of acid remaining in the solventextract can then be back-extracted with an alkali metal hydroxide orsalt solution.

"The most favourable selection of the temperature of the extractingoperation and of the compositions of the extractant, as regards both theamine and the solvent, will also be determined according to the givencondition of particular cases, e.g., the kind of acid, its concentrationin the original aqueous solution, the impurities present in thatsolution. The major aim in both the extracting and stripping operationswill be to achieve as favourable a distribution coefficent as possiblefor the distribution of the acid between the aqueous and organic phases.In the extraction operation, this has to be in favour of the extractant;in the stripping operation, in favour of the aqueous phase."

As stated above, the characterizing feature of said patents is thatback-extraction is performed at a temperature higher than that of theextraction. For certain acids, there is shown efficient extraction atabout room temperature. Back-extraction at about 100° C. provides for aback extract, the concentration of which is similar to, or even higherthan, that of the feed. In fact, a major part of citric acid productionin the world is based on this process, using tridodecyl amine as theprimary extractant and 1-octanol as the enhancer [Kirk-Othmer,Encyclopedia of Chemical Technology, 4th Ed., Vol. 6, p. 364].

The degree of product concentration in the TSP (the uphill pumpingeffect) depends strongly on the magnitude of the TS. The thermodynamicexplanation for the TS is not clear enough. One could suggest that asthe extraction process is exothermic, equilibrium is shifted backwardson temperature elevation. That would, however, be too simplistic. Thus,the most exothermic extraction is that of strong mineral acids, but noTS is found for their extraction. To the best of our knowledge, thiscomplex phenomenon was not fully explained in said patents, and no toolswere provided for predicting the magnitude of TS from the structure ofthe extracted acid.

The magnitude of the TS for extraction of various carboxylic acids by anextractant composed of 0.5 mol/kg trilauryl amine (Henkels Alamine 304)and 10% octanol in a kerosenic diluent have now been tested. The resultsare presented below in Table 1:

                  TABLE 1                                                         ______________________________________                                        The temperature sensitivity of carboxylic acid extraction by                    0.5 mol/kg Alamine 304 + 10% octanol in kerosene.                             The temperature sensitivity (TS) is presented as the                          distribution coefficient at 30° C., divided by that at 75°     C.,                                                                           at various equilibrium aqueous phase concentrations.                                        TS in Equilibrium with                                                                              Aqueous Solutions of (mol/kg)            Acid      pKa      0.05    0.2    0.3  0.475                                  ______________________________________                                        Maleic.sup.2                                                                            1.93     1.1     1.0    1.0  1.0                                      Oxoglutaric.sup.2 2.57 2.4 1.5 1.3 1.1                                        Malonic.sup.2 2.83 3.6 1.5 1.3 1.1                                            Tartaric.sup.2 3.01 3.4 3.2 2.7 2.4                                           Citric.sup.3 3.13 6.0 3.1 2.6 2.2                                             Malic.sup.2 3.22 4.0 4.3 4.0 4.0                                              Gluconic.sup.2 3.75 2.1 2.3 2.4 2.6                                           Lactic.sup.1 3.86 2.5 2.4 2.4 2.2                                             Succinic.sup.2 4.2 4.3 4.0 4.0 4.1                                            Glutaric.sup.2 4.4 3.9 4.5 4.5 4.4                                            Acetic.sup.1 4.76 2.3 2.4 2.4 2.4                                             Butyric.sup.1 4.81 2.1 2.0 2.0 1.8                                            Isobutyric.sup.1 4.84 1.9 1.5 1.4 1.1                                         Propionic.sup.1 4.87 1.7 1.5 1.3 1.1                                        ______________________________________                                         .sup.1 Monocarboxylic acid                                                    .sup.2 Dicarboxylic acid                                                      .sup.3 Tricarboxylic acid                                                

One can see that the TS may depend on the equilibrium concentration ofthe acid in the aqueous phase and that it varies significantly from oneacid to the other. No linear correlation is found, however, between theTS and the strength of the acid or another defined characteristicthereof. The strongest TS was found for citric acid at the lowconcentration of 0.05 mol/kg; some dicarboxylic acids show a higher TSthan their monocarboxylic analogues. That might indicate a tendency ofTS to increase with an increase in the number of carboxylic groups.Isolating this parameter from the others is difficult.

Extraction of strong mineral acids by ABE is very efficient, reachingstoichiometric levels already at equilibrium with dilute aqueoussolutions. That is true even for the weakest straight chain aliphaticamines, the tertiary ones reaching the stoichiometric extraction of 1mol of HCl per mol of amine in equilibrium with aqueous solutions ofabout 0.5%. High efficiency is also found in extracting strongcarboxylic acids having a pKa less than 2.5. The efficiency is, however,much lower on extracting weaker carboxylic acids by tertiary amines in akerosenic diluent. Said low efficiency is particularly pronounced in thelow concentration range. In order to avoid low yields of extraction,extraction enhancers are introduced into the extractant.

It is well-known that polar and protic compounds provide for enhancementof acid extraction by amines. These compounds may act as acidextractants by themselves, but are much weaker extractants than theamines. Extractants comprising amines and enhancers show synergisticeffects in most cases, i.e., acid extraction by such extractants is muchhigher than the added contribution of the components.

In the description of the invention herein, and to avoid confusion, theterm "primary extractant" will be used for long-chain amines used forextractions, and the term "enhancer" will be used for polar and proticextractant components, the extraction power of which is smaller thanthat of the primary extractant. Suitable enhancers are polar, andpreferably protic compounds, including alkanols, ketones, aldehydes,esters and ethers of various molecular weights.

Desired extractants should provide high efficiency in extraction(relatively low extractant volumes, a small number of extractant stagesand high yields), high selectivity, low water miscibility, low toxicity(particularly for food grade products), and efficient stripping of theextracted acid from the extract. The acid can be removed from theextract through interaction with an aqueous solution of a base to formits salt. In most cases, however, the acid is the required productrather than the salt, and acid recovery from the extract is performed byback-extraction with water or by distillation, where feasible.

As is known, high efficiency in extraction from the feed and highefficiency in stripping are conflicting requirements. Back-extraction ofthe extracted acid from a strong extractant requires high volumes ofwater and results in a very dilute aqueous solution of the acid(back-extract). The high cost of product concentration may make thewhole process impractical. Distillation from a strong extractantrequires high temperatures and may result in the decomposition of theacid and/or the extractant.

Extraction enhancers are polar and, preferably, protic compounds thathave very low extraction capacity on their own, but significantlyimprove the extraction efficiency of ABE. The enhancement is explainedby stabilization through salvation of the amine-acid ion pair. Octanolis used as an enhancer in the industrial TSP for production of citricacid.

Extraction enhancers have, however, an adverse effect on TSP, as thetemperature sensitivity decreases with an increase in enhancer content.Such an effect is shown below in Table 2:

                  TABLE 2                                                         ______________________________________                                        The dependence of the temperature sensitivity of citric acid                    extraction by amine-based extractant on amine concentration,                  enhancer (octanol) concentration, and on equilibrium aqueous                  phase concentration.                                                          The temperature sensitivity is presented as the ratio of                      distribution coefficient at 30° C. and 75° C.).                 Amine      Octanol D30/D75 at Aqueous Concentration                         mol/kg   mol/kg  0.02       0.5     1.5                                       ______________________________________                                        0.2      0.31    30.0       6.4     2.1                                         0.2 0.62 10.8 2.0 1.3                                                         0.2 2.0 4.9 1.3 1.1                                                           0.5 0.31 31.3 3.7 1.4                                                         0.5 0.62 4.6 1.5 1.1                                                          0.5 2.0 2.1 1.1 1.05                                                          1.0 0.31 10.5 1.2 1.07                                                        1.0 0.62 4.9 1.1 1.01                                                         1.0 2.0 1.8 1.08 1.03                                                       ______________________________________                                    

There is, therefore, a trade-off between extraction efficiency and themagnitude of the TS. Thus, aiming at a higher degree of productconcentration in the process leads to lower efficiency, particularly atthe low concentration end, resulting in lower recovery yields, i.e.,higher product losses. The absolute losses, expressed, for example, bythe product concentration in the raffinate, depend on the shape of thedistribution curve at the low concentration end. The proportional lossis mainly determined by the concentration of the acid in thefermentation liquor.

The TSP was implemented for citric acid recovery from fermentationliquors due to the unique, favorable combination of very hightemperature sensitivity (the highest reported so far) and the relativelyvery high concentration of citric acid in the fermentation liquor,typically 16-18%. Even at these unique conditions, the enhancer levelshould be reduced to a minimum. R. Wennerstern [J. Chem. Tech. Biotec.,No. 33B, pp. 85-94 (1983)] studied the effect of the various extractantparameters and concluded that hydrocarbons are the preferred diluents,as polar diluents reduce the temperature effect. Cooling below ambienttemperature or preconcentration of the fermentation liquor [U.S. Pat.No. 4,994,609] are required to avoid major product losses.

The above limitations brought Bauer, et al. to conclude, in 1989, that aTSP is not even economic for citric acid, and that displacement of theextracted acid by another acid (acetic) is preferable [Bauer, et al.,Ber. Bunsenges. Phys. Chem., Vol. 93, pp. 980-984 (1989)].

It is important to note at this juncture that ascorbic acid does notcarry a carboxyl group and therefore it is not a carboxylic acid, nor isit a mineral acid. Consequently, patents and disclosures which aredirected to processes for treating or recovering carboxylic and/ormineral acids do not include ascorbic acid within their scope.

According to its pKa, ascorbic acid is quite weak, being more than anorder of magnitude weaker than citric acid. Its low acidity and highhydrophilicity (since it carries 4 hydroxyl groups) reduce itsextraction efficiency.

Extraction efficiency is determined by the distribution coefficientdependance on the aqueous phase concentration (the shape of thedistribution curve). The distribution coefficient at the highconcentration end determines the maximal loading of the extractant, andthereby, the volume of the recycled extractant. The distributioncoefficient at the low concentration end determines the ability toapproach complete extraction, and thereby, the extraction yield. Forextraction of a component from a dilute feed, the yield of extraction isvery important. Reaching high yields in extracting from a dilute feed arelatively weak and highly hydrophilic acid, such as ascorbic acid,would require high enhancer levels.

Test results in Table 1 above show that the strongest temperaturesensitivity so far is found for citric acid, and that this temperaturesensitivity drops with a decreasing number of carboxyl groups. Nothingin these results, or in those found in the literature, indicates thatascorbic acid would show a higher temperature sensitivity than citricacid.

Even if ascorbic acid extraction had the temperature sensitivity ofcitric acid extraction, one would not consider its recovery from dilutesolutions in the TSP, due to the fact that at low enhancer levels, thelosses would be extremely high. On the other hand, at high enhancerlevels, the temperature sensitivity decreases. Thus, the major advantageof the process, i.e., recovering the product at a concentrationsubstantially higher than that of the fermentation liquor, would belost.

In light of the above, it was extremely surprising to discover that thetemperature sensitivity of ascorbic acid extraction by amine-basedextractants is very high and is maintained, even at high enhancerlevels. Based on this discovery, there is now provided, according to thepresent invention, a process for the recovery of ascorbic acid from anaqueous feed solution containing said acid at a concentration of lessthan 0.7 mol/kg, comprising extracting said ascorbic acid with awater-immiscible organic extractant composition comprising (a) at leastone secondary or tertiary alkyl amine in which the aggregate number ofcarbon atoms is at least 20, as a primary extractant, and (b) a polarextraction enhancer compound; wherein said extractant compositioncomprises at least 2 moles of said polar extraction enhancer compoundper one mole of primary extractant; separating said ascorbicacid-containing organic extractant composition from residual aqueoussolution, and subjecting said ascorbic acid-containing organicextractant composition to a stripping operation with aqueous solution ata temperature of at least 20° C. higher than the temperature at whichsaid extraction is carried out; whereby there is obtained an aqueoussolution of ascorbic acid in which the concentration of ascorbic acid ishigher than its concentration in said aqueous feed solution.

The process of the present invention is so effective that in preferredembodiments thereof as described hereinafter, said ascorbic acid can berecovered from an aqueous feed solution containing said acid at aconcentration of less than 0.5 mol/kg.

Extractants comprising relatively strong amines as the primaryextractant, show nearly no temperature sensitivity on the efficiency ofextracting strong mineral acids. It was, however, found that relativelyweak amines do show such effect. An example of such weak amines is thesterically-hindered, branched chain amines with branching on a carbonclose to the nitrogen atom [Eyal, et. al., Solvent Extraction and IonExchange, Vol. 9, pp. 195-236 (1991)]. These amines are weaker by morethan two orders of magnitude than straight chain amines, and weaker thanbranched chain amines with branching far from the nitrogen atom. Suchamines are too weak to extract most weak acids and are not suitable foruse as primary extractants in the present invention. For simplicity oflanguage, the term "branched chain amines" will be used here just forsterically hindered, relatively weak amines with branching close to thenitrogen atom.

Branched chain amines are too weak to extract many of the carboxylicacids, particularly hydroxycarboxylic acids. Straight chain amines aremuch more efficient, but complete extraction without resorting to highcooling costs requires the use of extraction enhancer. This isparticularly true for extraction from dilute feed solutions. Yet, thestronger is the enhancer and the higher its contents, the lower is thesensitivity of extraction efficiency to temperature. Thus, amine-basedextractants, comprising relatively strong enhancers at high proportionsof enhancers, show high efficiency in extraction, but lose most of theadvantage in back-extraction at higher temperature, according to U.S.Pat. No. 4,275,234.

According to the known practice, there have been suggested four mainoptions, as well as variations and combinations thereof:

a) Use of a weak enhancer or a strong enhancer, at a minimalconcentration required for extraction completion (non-optimal extractantcomposition in extraction, high extractant volume, many stages inextraction and relatively high losses). This option was chosen for thecitric acid production.

b) Increase the temperature span between extraction and back-extraction(expensive cooling and high viscocity in extraction, and expensiveheating and thermal degradation in back-extraction).

c) Distill at least part of the enhancer from the extract prior toback-extraction (high energy cost, limitation to volatile enhancers thatin most cases have relatively high solubility in the aqueous streams,requiring additional recovery operations).

d) Add to the extract an a-polar solvent that acts as extractionsuppressor, and removal of this solvent prior to the use of theregenerated extractant (low efficiency, high energy cost).

In contradistinction to the above options, a further preferred aspect ofthe present invention is based on the discovery that polar organiccompounds with steric hinderance of the polar group have, at aboutambient temperature, an enhancement effect similar to that of similarnon-hindered compounds, but lower enhancement effect at elevatedtemperature. As a result, efficient extraction is achievable usingamine-based extractants at about ambient temperature, in combinationwith convenient amounts of enhancer, while efficient back-extraction isachieved at elevated temperature, without resorting to unduly hightemperatures in back-extraction and/or high energy-consuming removal ofextractant components, either prior to back-extraction or after it.

Furthermore, it is well known that enhancer-containing extractantsprovide for more efficient extraction, but at the cost of reducedtemperature sensitivity of the extracting power. The advantage ofenhancer application in the extraction may be out-balanced by thereduced temperature sensitivity. Thus, for extraction of an acid from anaqueous feed of a relatively high acidity, particularly if incompleteextraction can be tolerated, non-enhanced (or slightly enhanced)extractants are preferred. On the other hand, in extraction from diluteaqueous solutions of acids, and particularly in extraction from aqueoussolutions of relatively high pH, an enhanced extractant is essential forefficient extraction (alternatively, a non-enhanced, very strong aminecan be used as a primary extractant, but stripping is impractical forsuch extractants).

In light of the above, there is now provided, according to preferredembodiments of the present invention, a process for the recovery ofascorbic acid from an aqueous feed solution containing said acid at aconcentration of less than 0.7 mol/kg, comprising extracting saidascorbic acid with a water-immiscible organic extractant compositioncomprising (a) at least one secondary or tertiary alkyl amine in whichthe aggregate number of carbon atoms is at least 20, as a primaryextractant, and (b) a sterically hindered, polar, organic, extractionenhancer compound having at least 5 carbon atoms, a basicity weaker thanthat of said primary extractant, and temperature-sensitive,extraction-enhancing properties; wherein said extractant compositioncomprises at least 2 moles of said extraction enhancer compound per onemole of primary extractant; separating said ascorbic acid-containingorganic extractant composition from residual aqueous solution, andsubjecting said ascorbic acid-containing organic extractant compositionto a stripping operation with aqueous solution at a temperature of atleast 20° C. higher than the temperature at which said extraction iscarried out; wherein said extraction enhancer compound both enhances theextracting power of said primary extractant composition and facilitatessaid temperature-sensitive stripping operation, and whereby there isobtained an aqueous solution of ascorbic acid in which the concentrationof ascorbic acid is higher than its concentration in said aqueous feedsolution.

In said preferred embodiments of the present invention, said stericallyhindered, polar, organic extraction enhancer compound is preferablyselected from the group consisting of alkanols, carboxylic acids,tertiary amines, or trialkylphosphates, having a sterically hinderingsubstituent attached to the carbon carrying said polar group, or to acarbon which is alpha, beta, or gamma to said carbon.

Polar, and particularly protic, organic compounds act as enhancers ofacid extraction by amines, due to their ability to solvate the amineacid ion pair formed on such extraction. Organic compounds suitable foruse as enhancers in the present invention have at least one such polaror protic group, the solvating properties of which are hindered by thestructure of the molecule. The polar group is preferably a hydroxyl, anester, an aldehyde, a carboxyl, a ketone, or an amine, or said polargroup can comprise a halogen, sulfur, nitrogen or phosphate atom. Thehindrance can be achieved through substitution of a hydrogen atom in thealkyl chain by an aliphatic group, i.e., branching on the carbon atomcarrying the polar group, or on a carbon which is alpha, beta, or gammato said carbon.

The enhancer should be a weaker base than the amine used as the primaryextractant in the extractant composite. On equilibrating it with a 0.1Maqueous HCl solution in a proportion that provides for enhancer to HClmolar ratio of 2, the aqueous phase pH will remain below 2. on a similarequilibration, with the amine acting by itself as the non-enhancedextractant, the pH of the aqueous phase increases to about 2.5 orhigher.

In addition to the primary extractant and the sterically-hindered,polar, organic enhancer compound, the extractant may comprise awater-immiscible, polar or non-polar solvent, for example, aliphatic oraromatic hydrocarbon, hydrocarbons carrying nitro or halo substituents,and alcohols.

In preferred embodiments of the present invention, said stericallyhindered, polar, extraction-enhancing compound is selected from thegroup consisting of secondary or tertiary alkanols, tris-2-ethylhexylamine, and tris-2-ethylhexyl phosphate.

The present invention also provides an extractant composition for use ina process for the recovery of ascorbic acid from an aqueous feedsolution containing said acid or a salt thereof, said compositioncomprising (a) at least one secondary or tertiary alkyl amine, in whichthe aggregate number of carbon atoms is at least 20, as a primaryextractant; and (b) a sterically-hindered, polar, organic extractionenhancer compound having at least 5 carbon atoms, a basicity weaker thanthat of said primary extractant, and temperature-sensitive,extraction-enhancing properties.

In preferred embodiments of the present invention, said extractioncomposition comprises at least 3 moles of said polar extraction enhancercompound per one mole of primary extractant.

In especially preferred embodiments of the present invention, saidstripping action effects the back-extraction of at least 80% of theascorbic acid contained in said organic extractant composition.

As will be described and exemplified hereinafter, one of the majoradvantages of the process of the present invention for the recovery ofascorbic acid is that, after said stripping operation, the remainingorganic extractant composition can be recycled, and further extractioncarried out with said recycled organic extractant composition providesyields of at least 90%, and preferably at least 95%, ascorbic acid.

The invention will now be described in connection with certain preferredembodiments with reference to the attached figures, so that it may bemore fully understood.

With specific reference now to the examples and distribution curvesshown in the attached figures in detail, it is stressed that theparticulars described and shown are by way of example and for purposesof illustrative discussion of the preferred embodiments of the presentinvention only, and are presented in the cause of providing what isbelieved to be the most useful and readily understood description of theprinciples and conceptual aspects of the invention. In this regard, noattempt is made to provide details of the invention more than isnecessary for a fundamental understanding of the invention, thedescription taken with the drawings making apparent to those skilled inthe art how the several forms of the invention may be embodied inpractice.

In the drawings:

FIG. 1 is a distribution curve for citric acid extraction by tricaprylylamine in kerosene, with various levels of octanol;

FIG. 2 shows distribution curves and temperature effect on differentacids;

FIG. 3 shows comparative distribution curves for ascorbic and citricacid; and

FIG. 4 is a distribution curve for ascorbic acid utilizing anon-sterically hindered extraction enhancer, as compared to asterically-hindered, polar, organic, extraction enhancer compound of thepreferred embodiments of the present invention.

Referring to FIG. 1, wherein Z is the acid/amine molar ratio in theorganic phase, it is seen that the extraction is enhanced by octanol,and the effect is particularly strong at the low concentration end.

FIG. 2 shows distribution curves for extraction by an extractantcomposed of 1.2 mol/kg tricaprylyl amine and 2.4 mol/kg octanol inkerosene. Extraction of ascorbic acid at 25° C. from an 0.2 mol/kgsolution can reach extractant loading of about 0.1 mol/kg. At 80° C.,however, by extrapolating the bottom curve, this extractant loading ofabout 0.1 mol/kg is equivalent to 0.8 mol/kg ascorbic acid in theaqueous phase.

The result indicates that in using this extractant over the temperaturegradient of 25-80° C., the uphill concentration factor for ascorbic acidis about 4. For citric acid and for succinic acid at these conditions,the factor is about 2. At this extractant composition, the TS forascorbic acid is higher than those for citric acid and for succinicacid. Comparison with succinic acid was included herein in case one wereto think that pKa is a factor in the results of the present invention,the pKa of succinic acid being the same as that of ascorbic acid.

As can be seen, however, the extraction for ascorbic acid is not yetsufficiently efficient and higher enhancer levels are preferred asdescribed hereinafter with regard to FIG. 3.

FIG. 3 illustrates distribution curves for extraction by an extractantcomposed of 1.2 mol/kg tricaprylyl amine (50%) and 3.8 mol/kg octanol(50%). The loading of the extractant in contact with 0.2 mol/kg ascorbicacid containing aqueous solution is about 0.5 mol/kg. Thus, increasingthe content of the enhancer and avoiding the kerosene strongly enhancedthe extraction, as compared to that shown in FIG. 2. The effect is evenmore impressive at the low concentrations end. The effect of the highenhancer level on the temperature sensitivity is surprisingly small. Aconcentration factor of about 4 can be reached on extraction at 25° C.and back-extraction at 96° C. Practically no temperature sensitivity isfound for citric acid extraction at these conditions.

Referring to FIG. 4, two extractants were tested. In both, the amine wastricaprylyl amine (Henkel's Alamine 336) and its concentration was 50w/w %. In one of the extractant compositions, the enhancer was anoctanol; in the other extractant composition, it was 3-ethyl-3-pentanol.In both cases, the enhancer content was 50% with no diluent having beenused.

Distribution of ascorbic acid between water and these extractants wastested at ambient temperature and at 75° C. The results are shown inFIG. 4. As can be seen, the extraction at ambient temperature wassimilar for both extractants, or even slightly higher in the use of3-ethyl-3-pentanol. At the elevated temperature, however, the extractantcomprising 3-ethyl-3-pentanol was less efficient.

From the results of the test exemplified in FIG. 4, it can be realizedthat using a sterically hindered polar organic compound having at least5 carbon atoms, a basicity weaker than that of the primary extractant,and temperature-sensitive, extraction-modifying properties as theextraction enhancer compound of the present invention, is indeedpreferred.

Referring once again to the teachings of U.S. Pat. No. 4,275,234, itwill be noted that several difficulties are indicated in the examples ofsaid patent:

In most examples, no enhancer was used in the extractant, or it is usedin a limited proportion of up to 5%. In Example 7, the extractantcomposition is 50% tri-tridecylamine and 50% nitrobenzene. Being a polarcomponent, nitrobenzene is quite efficient as an enhancer. An extractcontaining 9.3% citric acid was back-extracted with water (100 g per 100g of extract) at 60° C. (35° C. higher than the extraction temperature).Only 13% of the initial citric acid was back-extracted, forming a dilutesolution of 13% citric acid. Adding 150 g hydrocarbon to dilute theamine and the enhancer was needed to improve the back-extraction. Thisexample concluded that "the extract could not readily be back-extractedunless a hydrocarbon fraction was added to it." Addition of thehydrocarbon at the extraction step would have reduced its efficiency, asnon-polar solvents act contrary to the enhancers and could be referredto as extraction inhibitors.

Example 16 of said patent describes the back-extraction of oxalic acidfrom an extractant composed of 25% w/w dilaurylbenzyl amine, 69% w/wn-octane and 6% 1-n-octanol. For efficient back-extraction, 50 g ofn-octane were added to about 37 g of oxalic acid-containing extract.Thus, even at relatively low initial enhancer levels, substantialdilution by an extractant inhibitor was required. Only about 79% of theextracted acid is back-extracted at 80° C. Temperatures of 120-160° C.are recommended (Example 18).

The yield of lactic acid recovery from an initial solution comprising1.1 mol/kg acid was 95% (Example 13). Enhancer-free extractant was used.The yield for H₃ PO₄ recovery from an initial solution of 0.8 mol/kg was88% (Example 14). Here again, no enhancer was used. The extraction yieldfor citric acid in Example 5 was 95%, using an extractant comprising 5%enhancer (octanol).

In said patent, there also appears in Example 12 a description of theextraction of dilute lactic acid in which high amounts of enhancer areostensibly used with good results. According to the principles andtheory of the present invention, the results obtained in Example 12 ofU.S. Pat. No. 4,275,234 did not appear to be possible or correct. Inorder to clarify this point, the extraction of lactic acid from a 2%(0.22 mol/kg) solution and its stripping from the extractant wererepeated as in Example 12. The extractant was composed of 50% w/wtridodecylamine and 50% w/w of 1-n-octanol. The extraction was conductedat 25° C. and the stripping at about 96° C.

Extraction as in Example 12 (100 g aqueous, 40 g extractant, 3countercurrent stages) results in practically complete extraction of theacid to form an extract (loaded extractant) comprising 5% w/w lacticacid. Stripping as in Example 12 (40 g extract, 40 g water, 5countercurrent stages) results in an aqueous solution comprising 0.7 glactic acid in concentration of 1.8%. About two-thirds of the extractedlactic acid stays in the organic phase. Re-use of this organic phase inextraction from 2% lactic acid solutions results in low yields; not morethan 20% of the acid is extracted. Increasing the number of stages inextraction has only a small effect. Near complete stripping and thushigh yield in re-use of the organic phase requires about 150 g water per40 g of extract, and 6-7 countercurrent stages. The lactic acid in thiscase is obtained in a dilute solution of about 0.5% w/w.

Thus, using an extractant comprising about 4 moles of enhancer per moleamine provides for nearly complete extraction of lactic acid from adilute solution of 0.22 mol/kg, but on stripping, a high proportion ofwater is required and the acid is diluted 4 times, compared to itsconcentration in the feed. The cost of concentrating this solution isenormous.

Using the same extractant for extracting ascorbic acid from 0.22 mol/kgsolution, 65 g of extractant per 100 g aqueous solution and 5-6countercurrent stages, are required to reach an extraction yield of atleast 95% at 25° C.

Stripping the extract at 96° C. with 35 g water results in an aqueoussolution comprising 0.6 mol/kg ascorbic acid and an organic phasepractically free of ascorbic acid. Re-use of this organic phase inextraction provides an extraction yield of at least 95% at the aboveconditions.

Thus, while in the case of lactic acid, practically complete extractionwith recycled extractant results in a lactic acid product diluted 4times compared with the feed, in the case of ascorbic acid at the sameconditions and with similar extractant, practically complete extractionwith recycled extractant results in ascorbic acid product solutionconcentrated 3 times compared with the feed.

Therefore, it is clear that one following the teachings of U.S. Pat. No.4,275,234 and repeating the examples contained therein would come to theinescapable conclusion that the process taught therein is not suitablefor the commercial production of ascorbic acid. Furthermore, said patentcertainly does not teach or suggest the use of a stearically-hindered,polar, organic, extraction enhancer compound as described and claimedherein.

It will be evident to those skilled in the art that the invention is notlimited to the details of the foregoing illustrative examples and thatthe present invention may be embodied in other specific forms withoutdeparting from the essential attributes thereof, and it is thereforedesired that the present embodiments and examples be considered in allrespects as illustrative and not restrictive, reference being made tothe appended claims, rather than to the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are therefore intended to be embraced therein.

What is claimed is:
 1. A process for the recovery of ascorbic acid froman aqueous feed solution containing said acid at a concentration of lessthan 0.7 mol/kg, comprising:extracting said ascorbic acid with awater-immiscible organic extractant composition comprising (a) at leastone secondary or tertiary alkyl amine in which the aggregate number ofcarbon atoms is at least 20, as a primary extractant, and (b) a polarextraction enhancer compound; wherein said extractant compositioncomprises at least 2 moles of said polar extraction enhancer compoundper one mole of primary extractant; separating said ascorbicacid-containing organic extractant composition from residual aqueoussolution, and subjecting said ascorbic acid-containing organicextractant composition to a stripping operation with aqueous solution ata temperature of at least 20° C. higher than the temperature at whichsaid extraction is carried out; whereby there is obtained an aqueoussolution of ascorbic acid in which the concentration of ascorbic acid ishigher than its concentration in said aqueous feed solution.
 2. Aprocess for the recovery of ascorbic acid as claimed in claim 1, whereinsaid extractant composition comprises at least 3 moles of saidextraction enhancer compound per one mole of primary extractant.
 3. Aprocess for the recovery of ascorbic acid as claimed in claim 1, whereinsaid stripping action effects the back-extraction of at least 80% of theascorbic acid contained in said organic extractant composition.
 4. Aprocess for the recovery of ascorbic acid as claimed in claim 1,wherein, after said stripping operation, the remaining organicextractant composition is recycled.
 5. A process for the recovery ofascorbic acid as claimed in claim 4, wherein further extraction carriedout with said recycled organic extractant composition provides yields ofat least 90% ascorbic acid.
 6. A process for the recovery of ascorbicacid as claimed in claim 4, wherein further extraction carried out withsaid recycled organic extractant composition provides yields of at least95% ascorbic acid.
 7. A process for the recovery of ascorbic acid asclaimed in claim 1, wherein said aqueous feed solution contains saidacid at a concentration of less than 0.5 mol/kg.
 8. A process accordingto claim 1, for the recovery of ascorbic acid from an aqueous feedsolution containing said acid at a concentration of less than 0.7mol/kg, comprising extracting said ascorbic acid with a water-immiscibleorganic extractant composition comprising:(a) at least one secondary ortertiary alkyl amine in which the aggregate number of carbon atoms is atleast 20, as a primary extractant, and (b) a sterically hindered, polar,organic, extraction enhancer compound having at least 5 carbon atoms, abasicity weaker than that of said primary extractant, andtemperature-sensitive, extraction-enhancing properties; wherein saidextractant composition comprises at least 2 moles of said extractionenhancer compound per one mole of primary extractant; separating saidascorbic acid-containing organic extractant composition from residualaqueous solution, and subjecting said ascorbic acid-containing organicextractant composition to a stripping operation with aqueous solution ata temperature of at least 20° C. higher than the temperature at whichsaid extraction is carried out; wherein said extraction enhancercompound both enhances the extracting power of said primary extractantcomposition and facilitates said temperature-sensitive strippingoperation, and whereby there is obtained an aqueous solution of ascorbicacid in which the concentration of ascorbic acid is higher than itsconcentration in said aqueous feed solution.
 9. A process according toclaim 8, wherein said sterically hindered, polar, organic, extractionenhancer compound is selected from the group consisting of alkanols,carboxylic acids, tertiary amines, or trialkylphosphates having asterically hindering substituent attached to the carbon carrying saidpolar group, or to a carbon which is alpha, beta, or gamma to saidcarbon.
 10. A process according to claim 9, wherein said substituent isan aliphatic group.
 11. A process according to claim 8, wherein saidextraction enhancer compound is selected from the group consisting ofsecondary or tertiary alkanols, tris-2-ethylhexyl amine, andtris-2-ethylhexyl phosphate.
 12. A process according to claim 1, whereinthe aqueous feed solution of ascorbic acid is obtained by fermentation.13. An extractant composition for use in a process for the recovery ofascorbic acid from an aqueous solution containing said acid or a saltthereof, said composition comprising:a) at least one secondary ortertiary alkyl amine in which the aggregate number of carbon atoms is atleast 20, as a primary extractant; and b) a sterically hindered, polar,organic compound having at least 5 carbon atoms, a basicity weaker thanthat of said primary extractant, and temperature-sensitive,extraction-enhancing properties.
 14. An extractant composition asclaimed in claim 13, further comprising a water-immiscible, organicsolvent.